The invention relates generally to prosthetic mechanical heart valves. More particularly, the invention relates to a method and an apparatus for inserting occluders in prosthetic heart valves.
The human heart acts as a pump and includes valves which regulate blood flow in one direction and prevent blood back flow. The heart valves can be damaged or malfunction. A common surgical technique for returning the heart to normal pumping operation is to replace a malfunctioning valve with a prosthetic heart valve. A typical prosthetic heart valve includes an annular housing which defines an orifice for blood flow. The housing supports one or more occluders (also known as leaflets). The occluders open and close in response to changes in blood pressure on either side of the valve, allowing blood flow in one direction. Prosthetic heart valves are manufactured in various sizes to accommodate variations in the size of the heart and valves in different patients.
Prosthetic heart valves are available in several different types. One type of prosthetic heart valve has a single occluder, generally placed off-center so that it pivots in response to changes in blood pressure. Another type of valve, the bi-leaflet valve, includes two occluders or leaflets, each pivotally mounted. A leaflet of a valve typically includes two generally opposed ears or projections which are integral to the leaflet. Each ear fits into corresponding slots or pivot supports on the orifice housing which allows the leaflet to pivot.
The orifice housing of a typical bi-leaflet valve is annular or ring-shaped, and has a generally circular cross-section. The interior surface of the orifice housing has a pair of flat wall sections opposite each other, i.e., diametrically opposed. These flat wall sections are referred to as the orifice flats. The flat surfaces are both secants of the annular shape, cutting off part of the circle on the interior surface of the orifice housing. The result is that this type of valve housing has its shortest interior diameter perpendicular to the orifice flats. Generally, the pivot supports for the leaflets are located within the orifice flats.
One method for installing leaflets in a valve housing is to apply a force to the housing which deforms the housing so that the pivot supports are spread far enough apart to permit the leaflets to be positioned so that the ears align with the pivot supports. When the force is removed, the ears are pivotally mounted in the pivot supports and the housing returns to its annular shape. Valve housings are generally manufactured from materials having sufficient elasticity to allow for some distortion of the housing, e.g., pyrolytic carbon and pyrolytic carbon-coated graphite. After leaflet installation, a stabilizing ring may be shrunk-fit around the exterior surface of the housing to stabilize it and make sure it retains its circular cross-sectional shape.
One technique for installing leaflets in the pivot supports is to engage a set of pins or shoes with the interior surface of the valve housing. The pins or shoes are placed against the housing near the pivot supports. By applying a force to the pins or shoes, the housing is deformed to provide clearance to install the leaflets. After the leaflets are placed into the pivot supports, the force to the pins or shoes is removed and the pins or shoes are retracted.
The insertion of occluders into the valve housing is known as one of the more difficult aspects of designing and manufacturing prosthetic heart valves. The installation of the occluders must meet several requirements. First, the process must provide sufficient occluder capture within the valve housing to prevent leaflet release. If a leaflet comes loose from a prosthetic heart valve after implantation in a patient, the loose leaflet may cause blood vessel embolization, posing a serious health risk for the patient. Therefore, the leaflets must be securely installed in the valve housing and must remain in place, even under high pressures.
Next, the valve housing must be sufficiently stiff to retain the leaflets in place during the pumping operation of the heart. If the housing is too flexible, when the heart flexes during its pumping cycle the leaflets may pop out of the pivot supports.
Another requirement is for the installed leaflets to open and close reliably. If a leaflet sticks, or is jammed open or closed, the result may create a serious health risk for the patient. A stiff valve housing helps to prevent binding between the occluders and the valve housing as the heart flexes, improving reliability.
On the other hand, a stiff valve housing makes leaflet insertion more difficult. The more flexible the housing, the easier it is to deform the housing and move the orifice flats a distance sufficient to allow occluder insertion. However, if the force applied to deform the housing exceeds the fracture stress limitations of the housing material(s), the housing may develop stress fractures or cracks. Generally, valve bodies with cracks must be discarded; therefore, preventing cracks can greatly reduce the expense of valve manufacture. Consequently, the housing must be made from materials which allow these counter-balancing design factors to be met.
As stated above, pyrolytic carbon is a material commonly used for manufacturing the valve housing. The problems of holding the leaflets in place, reliable leaflet opening and closing, and cracking are compounded by certain constraints associated with pyrolytic carbon manufacturing. Two possible approaches for pyrolytic carbon manufacturing are "mandrel" and "substrate" manufacturing. "Mandrel" refers to a core around which the pyrolytic carbon is dip-cast or otherwise shaped. The mandrel is removed after shaping. In substrate manufacturing, the pyrolytic carbon is also shaped around a core, but the core is not removed after shaping.
Mandrel products typically have stress concentration features on the outer surface of the valve housing as a result of coating over pivot detail from the mandrel. Substrate products typically are very stiff due to the greater sectional properties provided by having the substrate present under the pyrolytic carbon coating. These constraints tend to make leaflet insertion more difficult in pyrolytic carbon valves. This is particularly true for the smaller valve sizes, which generally show the greatest stiffness among a family of valve sizes.
Consequently, there is a need for leaflet insertion which maximizes the flat-to-flat deflection of the valve housing within the constraint of its material strength, while not damaging the valve in any way. Such an approach should work for all valve sizes with minimal changes, and work for various valve designs. The method should also be easy to use and reliably repeatable, have little or no dependence on the skill of an operator for success, and be easy to implement in the manufacturing process.